A mass spectrometry (MS) system in general includes an ion source for ionizing components of a sample of interest, a mass analyzer for separating the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. Typically, the mass spectrum is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. The mass spectrum may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized.
To elucidate additional information regarding a sample, the MS system may be configured for carrying out tandem MS, or MS-MS, experiments. In this case, selected ions produced by the ion source, or “parent” ions, are dissociated into fragment ions (or “daughter” ions) in a collision cell. Parent ions not dissociated in the collision cell as well as fragment ions may then be transmitted into the mass analyzer to produce mass spectra. Tandem MS may be implemented in a triple quadrupole (or QQQ) MS system, which includes three quadrupole devices in series. The first quadrupole is utilized for mass selection, the second quadrupole is an RF-only device enclosed in a gas chamber and utilized as the collision cell, and the third quadrupole is utilized as the mass analyzer. Tandem MS may also be implemented in a quadrupole time-of-flight (or qTOF) MS system, the main difference being that the mass analyzer is a TOF analyzer instead of a quadrupole device.
The MS system may include an ion trap configured for storing or accumulating ions prior to transmitting the ions to downstream processes. In particular, an ion trap is capable of scanning (ejecting) the ions out of the ion trap on a mass-selective basis (i.e., according to m/z ratio) using known techniques. Thus an ion trap may also be utilized to sort ions and may be considered as part of a hybrid system similar to a QQQ or qTOF system.
In an ion trap, a radio frequency (RF) voltage on the trap electrodes generates a time-varying electric field in the trap interior that confines ions having a desired range of m/z ratios. Ions of a selected m/z ratio may then be ejected from the trap by known methods such as instability ejection or resonant ejection. When ions are ejected from an ion trap they may exit the trap over a range of phase angles relative to the phase of the main RF trapping voltage. The RF phase at the time of ejection may vary over a range of, for example, sixty degrees. Correspondingly, the trap instantaneous voltage at the time of ejection may vary from about half of the peak RF voltage down to zero. Ions ejected from the trap, even though they all may have the same m/z ratio, then have that same range of energies relative to any DC reference potential outside of the trap. Ejecting ions over a wide energy distribution may be undesirable when coordinating the ion ejection with downstream processes, such as beam focusing or injection into a collision cell or mass analyzer. In addition to the problem of energy distribution, ions typically are not well collimated when exiting the trap. The typical angular spread of the ejected ions may be too large for many applications. This may especially be true if ion optics lenses are added at the trap output, where changes in divergence from focusing will, in general, interact with the energy distribution of the ions and the rf phase at the time of ion ejection.
Therefore, there is a need for systems, devices and methods for focusing and adjusting the energy of ions ejected from an ion trap.